Functional studies combined with the 1.35 E crystal structure of Pin1 show a preference for a phosphorylated Ser/Thr immediately N-terminal to proline in its target selection. In efforts to visualize these interactions and clarify the mechanism of Pin1 mediated peptide bond isomerization we have begun co-crystallization of Pin1-peptide complexes as well as soaks of Pin1 crystals with phospho-Ser-Pro dipeptides. Ubiquitin ligases (E3) determine the selectivity of ubiquitin-mediated protein degradation. Structural studies of these ubiquitin ligases are crucial in elucidating their reaction mechanisms and most importantly their mode of target selection. We have grown crystals of the Nedd4-like ubiquitin ligase hect-domain in space group P1.